Regenerative pyrolysis reactors may be utilized in pyrolyzing or cracking hydrocarbons. Regenerative reactor cycles are either symmetric (same chemistry or reaction in both directions) or asymmetric (chemistry or reaction changes with step in cycle). Examples of these reactors and the associated processes are described in U.S. Pat. No. 2,319,679; 2,678,339; 2,692,819; 3,024,094; 3,093,697; and 7,943,808. As one of the steps in the cycle, combustion can be used for regenerating reactors to perform cyclic, high temperature chemistry.
The process typically involves a heating step (e.g., regeneration step) and a pyrolysis step in different portions of a cycle. The heating step includes exothermic reactions, e.g., by conducting fuel and oxidant to a reaction zone, combusting the fuel and oxidant, and then conducting the combustion products away from the reaction zone. During the pyrolysis step, a feed containing hydrocarbons is conducted through the reaction zone, thereby pyrolyzing the hydrocarbons in the feed and conveying heat from a reactor bed or other source. Some regenerative pyrolysis reactors deliver fuel and/or oxidant directly to the combustion zone without having that stream pass through reactor beds that preheat the stream. The fuel and/or oxidant is typically introduced via nozzles, distributors, or burners that extend within the reactor using means generally perpendicular to the reaction flow direction and usually through the reactor vessel side wall. For example, during the heating step in a conventional Wulff cracking furnace, air flows axially through the regenerative bodies, and fuel is introduced via nozzles that extend within the side of the reactor, to combine with air (combusting and releasing heat) in an open region between regenerative bodies.
High-severity pyrolysis in regenerative pyrolysis reactors can result in high selectivity to acetylene that may be utilized for many dimensions of chemicals growth from natural gas and other hydrocarbon feeds. Accordingly, the capabilities of regenerative pyrolysis reactors or reverse-flow regenerative pyrolysis reactors suggest that these reactors may achieve the desired reaction conditions at extreme temperatures (e.g., greater than (>) 1700° C.) in a cost effective manner. That is, the higher temperatures within the reactor are preferred for high conversion of feeds to C2 unsaturates. Particularly, conversions of methane greater than 50% (and reasonable selectivity) require temperatures >1400° C. and millisecond residence times. Longer residence times favor the formation of coke, an unwanted byproduct. For heavier hydrocarbons, the higher temperatures (e.g., >1400° C.) lessen heavy hydrocarbon co-product production, thereby simplifying the recovery stages of the process (e.g., fewer heavy co-products require less equipment to separate the co-products from the desired streams).
Operation of high severity hydrocarbon cracking processes and regenerative pyrolysis reactors involves various competing operational and engineering challenges. For example, as noted in U.S. Patent Pub. App. Nos. 2010/0290978 and 2010/0292523, the high temperatures can exceed the long term viability of conventional apparatus, including conventional ceramics. That is, the reactor components have to be thermally stable at these high temperatures.
While the high temperatures enhance selectivity to C2 unsaturates, the rapid kinetics of high temperature operation mandate short residence times (e.g., milliseconds). As a result, high flow velocities of the streams through the reactor or use of shorter reactors are needed to provide these short residence times. Unfortunately, the high flow velocities increase pressure drop, which is yet another challenge for the operation of these reactors. Accordingly, the conventional approach is to reduce residence time by shortening the length of the reactor bed to provide short residence time without unduly high flow velocity. However, shortening the reactor bed while maintaining high productivity (e.g., volumetric flow rate) results in lowering the length to diameter ratio for the reactor, causing significant flow distribution challenges.
In addition, conventional techniques fail to adequately address the heat transfer rate within the reactor. The heat transfer rate involves convective heat transfer and radiant heat transfer, which are present for certain operating temperatures. Convective heat transfer has a rate that is proportional to surface area (e.g., wetted area, or transfer area per bed volume), while radiative heat transfer is emitted by gases and components within the reactor. Regenerative reactor efficiency is enhanced by increasing the rate of heat transfer. However, pressure drop (e.g., momentum transfer) is also proportional to the wetted area. Thus, conventional attempts to enhance reactor efficiency by increasing wetted area also result in increased pressure drop.
Accordingly, it is desired to enhance the flow of fluids through reactor in a manner that increases the efficiency of the process. Further, it is desired to have a reactor configuration that provides high heat transfer rates and short residence times without imposing process-limiting constraints of higher pressure drop. Also, it is desired to utilize the convective properties along with the radiative properties for certain portions and process flow components based on the temperature profile within the reactor to further enhance the operation of the process.